Broadband Integrated Services Digital Network or B-ISDN is a new telecommunication technology developed by the telecommunications carrier industry for both data transmissions (computer) and telecommunications (telephone). It is conceived as a carrier service to provide high speed communications to end users. The technology selected to deliver B-ISDN service is called "Asynchronous Transfer Mode" or ATM. The acceptance of ATM comes from the fact that ATM handles all the different kinds of communication traffic, such as voice, data, image, video, high quality sound, multimedia and others. ATM traffic can be carried in both LAN (Local Area Network) and WAN (Wide Area Network) network environments and hence promises a seamless inter-working between the two.
ATM handles all types of traffic adequately and in an integrated way. This means that, instead of having a proliferation of many specialized kinds of equipment for different functions, it is now possible to have a single type of equipment and network which will do everything.
Communication technologies have realized considerable progress and many potential applications that were not possible before are now becoming accessible and attractive. One of the driving forces behind ATM development is the need for a communication network architecture that can take advantage of the the following changes: high speed rates can now be sustained with very low bit error rates with the maturing of new transmission media and especially of optical fibers: the very real user demand for data communication services and for ever faster services; silicon chip technology has improved to the point where very fast switching systems can be built; the general belief that integrated packet (or cell) based switching systems are significantly lower in cost than Time Division Multiplexed (TDM) type systems; and the development (again due to improvement in silicon technology) of much faster and lower-cost computer hardware makes many new applications possible that were not economically feasible before.
The principle key concepts of ATM are as follows:
Cells: all information (voice, image, video, data . . . ) is transported through the network in very short, fixed length (48 data bytes plus a 5-byte header) blocks called cells. The ATM cell size was determined by the CCITT, now called the International Telecommunication Union (ITU), as a compromise between voice and data requirements.
Routing: information flow along paths (called "virtual channels") is set up as a series of pointers through the network. Each cell header contains an identifier that links the cell to the correct path towards its destination. Cells on a particular virtual channel always follow the same path through the network and are delivered to the destination in the same order in which they are received.
Hardware-Based Switching: ATM is designed so that simple hardware based logic elements may be employed at each node to perform the switching. On a link of 1 Gbps, a new cell arrives and a cell is transmitted every 0.43 microseconds. There is not a lot of time decide what to do with an arriving cell.
Adaptation: at the edges of the network, data frames are broken up into cells. Continuous data streams such as voice and video are segmented into cells. At the destination side of the network the user data frames are reconstructed from the received cells and returned to the end user in the form (data frames etc.) that they were delivered to the network. This adaptation function is considered part of the network but is in a higher layer function, which is called ATM Adaptation Layer (AAL).
Error Control: the ATM cell switching network checks cell headers for errors and simply discards cells in error. The adaptation function AAL is external to the switching network and depends somewhat on the type of traffic but for data traffic it usually checks for errors in data frames received and if one is found it discards the whole frame. At no time does the ATM network attempts to recover from errors by the re-transmission of information. This function is up to the end user devices and depends on the type of traffic being carried and the adaptation layer being used.
Flow Control: an ATM network typically does not implement internal flow control. The required processing logic is generally considered too complex to be accommodated at the speeds involved. Instead ATM has a set of input rate controls that limit the rates at which traffic is delivered to the network.
Congestion Control: When a link or node becomes congested, cells are discarded by the network until the problem has been relieved. Some (lower priority) cells can be marked such that they are the first to be discarded in the case of congestion. Connection endpoints are not notified when cells are discarded. It is up to the adaptation function or higher-layer protocols to detect and recover from the loss of cells (if necessary and possible).
In order to make an ATM network practical it is necessary to adapt the internal network characteristics to those of the various traffic types that will use the network. This is the of the adaptation layer. It would have been possible to leave this function to end-user equipment suppliers but that could mean that many different systems of voice or video coding (many incompatible with one another) would come into use. The function of the adaptation layer is to provide generalized inter-working across the ATM network. In the case of data, the AAL takes frames (blocks) of data delivered to it, breaks them up into cells and adds necessary header information to allow rebuilding of the original block at the receiver.
Recommendations of the International Telecommunication Union (ITU) have defined four generic classes of network traffic that need to be processed differently by an ATM network. These classes are class A for constant rate voice and video applications, class B for isochronous voice and video traffic with variable bit rate services, class C for traditional data traffic as known in an SNA or X.25 network, and class D intended to support connectionless networking protocols such as TCP/IP.
The AAL function implements a number of end-to-end protocols across the ATM network to provide support for end users of the four identified service classes. There are five different AAL types called AAL-1 to AAL-5. AAL-5, which offers very high performance and is available in every workstation provides services for the support of data, video, signaling traffic and can also be used for voice.
Unfortunately, no traffic class has been provided for the transmission of asynchronous data wherein each character contains several bits of data (generally 5 to 9 bits), one START bit and one or two STOP bits. Such asynchronous data is transmitted on a serial bus (such as the RS 232 link) which exists on all personal computers.
Several techniques are nevertheless used to transport Start-Stop protocols over an ATM network. One of them consists in sampling the sequence of asynchronous characters to get a stream of synchronous data bits. But, the sampling frequency must be at least 4 times the speed defined on the interface in order to get correct data on the other side of the network. The main drawback of such a technique is therefore the necessary overhead and the fact that the required bandwidth is used even if no data is present.
Another technique is the use of a Packet Assembly/Disassembly (PAD) facility wherein the asynchronous data is encapsulated with X.28 protocol before being converted to X.25 protocol by the PAD (X.3 protocol). Such a technique requires significant overhead and introduces a delay in the transmission. Furthermore, X.25 protocol is applied for each user address and no data streaming mode is available.
There is another technique using the so-called Frame Relay Access Device (FRAD) enabling the asynchronous data to be transported over an ATM network. But, it is similar to the PAD technique and has the same drawbacks.